Concrete is a very widely used construction material with high strength and good durability. In addition to aggregates and water, it also contains Portland cement as a hydraulic binder, which produces strength-forming phases by solidifying and curing in contact with water. Concrete based on Portland cement clinker is thus one of the most important binders worldwide.
By adding various additives such as, e.g., granulated blast-furnace slag (gbfs), fly ash, natural pozzolans, calcined clays or ground limestone to Portland cement, Portland composite cements having different properties can be produced. At the same time, the specific emission of CO2 will be reduced in the production of cement by substituting the cited additives for Portland cement, because during the production of Portland cement clinker about 0.9 tons of CO2 per ton of Portland cement clinker will be emitted by the calcination of the raw materials and from the oxidation of the fuels in the rotary tubular kiln. The addition of additives to Portland cement has been an established practice for more than 100 years and is regulated in numerous cement and concrete standards.
The addition of ultra-fine additives, such as microcement or microsilica, to Portland cement is used to enhance the durability and the strength of the resulting concrete. The traditional way of formulating high durability and strength and very high durability and strength concrete is based on using ordinary Portland cement with ultra-fine additions as a binder. The ultra-fine additions commonly used are often conditioned in bags and mixed into the Portland cement by hand at the respective concrete plant or at the construction site, which involves safety risks as well as quality uncertainties. The ultra-fine additions can also be stored in a dedicated silo on a concrete plant and introduced automatically, but this involves the need of very specific industrial equipment that represents additional investments.
Another disadvantage of using ultra-fine additives in a cement mixture is the elevated water demand, since the water demand rises with increasing fineness of the ultra-fine additives.
It is commonly assumed that the durability and the strength of the resulting concrete, such as the compressive strength, strongly depends on the proportion of the ultra-fine additions in the cement mixture, namely that the higher the content of ultra-fine additions is, the better the durability and strength of the concrete is. Producing ultra-fine additions is costly due to the elevated grinding effort. Therefore, the cost of the cement mixture rises with an increasing content of ultra-fine additions.
Therefore, it is an object of the present invention to provide a cement mixture that overcomes the above drawbacks. In particular, it is an object of the invention to provide a cement mixture that allows an easy and reliable manufacture of concrete at a stable quality level. It is a further object of the invention to reduce the water demand without impairing the workability of the concrete mass. Further, the resulting concrete shall have excellent durability and strength at a reasonable production cost.